RAINCHECK

Any voucher or note from a merchant to a customer to provide an item the merchant has run out of at a later date for the item's current price, or (in a more literal sense) to provide a service at a later date.

In social interactions, a polite way to turn down an invitation, with the implication one is simply postponing it and that another time would be acceptable.

"I can't go with you to the museum this Saturday, but can I take a rain check and go some other day?"